Molecular Structures Using VSEPR



Molecular Structures Using VSEPR

In this lab activity, you will determine the Lewis structures and the three-dimensional geometries of molecular species given only their molecular formulas. This will help you perfect your skill at drawing Lewis structures and help you become better acquainted with molecular geometry by constructing models of the various geometries. You may work in groups, but each student will be responsible for his/her own drawings.

You must do the following for each species on your list below (include a table of items 1, 2, 4, and 5, as shown on p. 2). This should all be done in your lab notebook, as there is not a separate hand-in assignment for this experiment.

  1. Draw the Lewis structure. Include all lone pairs and indicate non-zero formal charges on the
  2. Determine the electron pair arrangement around each central Number central atoms where necessary.
  3. Construct a virtual model of the species at shapes/latest/molecule-shapes_en.html (“Model” option, add the appropriate number of bonds and/or lone pairs).
  4. Draw a 3D representation of the molecule from the model using the conventions discussed in your textbook and in class.
  5. Give the name of the molecular geometry or shape around each central atom. Number central atoms where



NH3      (1 ´ 5 + 3 ´ 1 = 8 valence e)

  1. Lewis structure:



  1. Electron pair arrangement: Tetrahedral

3 bonding pairs + 1 lone pair = 4 pairs → tetrahedral e pair arrangement

  1. Construct a molecular model at the website listed DO NOT SKIP THIS STEP.
  2. 3D drawing:




  1. Molecular geometry: Trigonal pyramidal
    • bonding pairs + 1 lone pair → trigonal pyramidal molecular geometry


C3H8    (3 × 4 + 8 × 1 = 20 valence e)

  1. Lewis Structure:




  1. Electron-pair arrangement: Tetrahedral around each C
    • bonding pairs around each C → tetrahedral e pair arrangement for each C


  1. Construct a model. For organic molecules such as this with more than one central atom, draw the Lewis structure at You will first need to click on the trash can to delete the default (caffeine) molecule. Build the molecule using the drawing tools in the left panel, then click “2D to 3D” to see the 3D model in the right


  1. 3D drawing: For organic compounds, orient the carbon chain so that as many atoms as possible lie in the same plane (in this case, the three carbons and one H on each end C). This is an easier perspective to






  1. Molecular geometry: Tetrahedral around each C

4 bonding pairs around each C → tetrahedral molecular geometry for each C




Unless otherwise stated, assume the first atom listed is the central atom.


  1. CH3Br SbCl5
  • ICl + BrF3
2                                                                                                                                                                4
  • NO ICl
3            2            2
  • BF3 CH CH CO (bonding as in formula)
  1. CS2 C5H10 (pick one structure)


For #9-10, there are a variety of Lewis structures (isomers) that could be drawn. Arrange the atoms so that the octet rule is obeyed for all non-hydrogen atoms. Note that the hydrogen atoms will be terminal atoms. In your table, specify the electron pair arrangement and molecular geometry about each unique non-terminal atom.



Suggested Table Format for Notebook:


Molecular Formula Lewis Structure Electron Pair Arrangement (around central


3D Drawing Molecular Geometry (around central




Grading: 2 points per species.


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